If you do a search the number one problem or concern for running OpenBSD is performance. And I dont think much has changed. I dont doubt there are certain domain where you would value security over everything.
OpenBSD had pretty poor support for SMP for quite a while, but lots of work has gone into fixing that in the last decade. That plus how much smaller the OpenBSD footprint is than, say, Linux, and you might find these days it performs pretty well.
Is packet filtering still single threaded? I remember pf could only could only use a single core at a time which made filtering more than 1 Gb/s problematic.
It's threaded as of recently. Before that the throughput obviously hinged on single-core performance. 1 Gbit/s sounds like lower tier laptop spec to me. My 2015 low-power Celeron test rig could do ~700 Mbit/sec through a medium complexity PF ruleset on OpenBSD versions where PF was still single-threaded.
I believe it is. I will, however, admit that while I use OBSD extensively for networking devices (routers, etc.), I haven't had much call for multigig networking on them. I will say that it will easily do 1G routing+PF+VLAN on rather modest hardware (J4105, dual i211 ports).
